Thermoelectric properties and the effect of biaxial strain and external electric fields on the electronics of novel 2D Lace-like O-Pd2Q3 (Q= S, Se) monolayers

Abstract

Inspired by the experimental synthesis of the novel 2D O-Pd2Se3 monolayer, we performed density functional theory calculations to reveal detailed investigations of the electronics, and thermal properties of 2D O-Pd2Q3 (Q= S, Se) monolayers. The O-Pd2S3 and Pd2Se3 are semiconductors with indirect band gaps of 0.43 eV and 0.33 eV, respectively. Through strain engineering, the bulk modulus B (N/m) and young's modulus Y (N/m) are also determined to acquire a deep understanding of the elasticity of the monolayers. We find that the bandgaps tend to increase with increasing tensile biaxial strain (+ε%), and decrease with increasing compressive biaxial strain (-ε%). In particular, the O-Pd2Se3 monolayer starts to manifest a metallic feature at -5%. More importantly, we note that at room temperature, and under biaxial strain, O-Pd2S3 (O-Pd2Se3) has a high electron (hole) carrier mobility up to ∼ 766 cm2 V−1 s−1 (∼ 106 cm2 V−1 s−1). We have also tuned the band gaps of the monolayers via the application of an external electric field, along the z-direction. We find that, in the [-2.5, +2.5] V/Å range, such external electric fields reduce the bandgaps in both monolayers, transforming Pd2Se3 into a metal at a strength of ± 2.5 V/Å. Additionally, we have perceived that the thermoelectric (TE) properties of these materials exhibit an anisotropic behavior at ambient and higher temperatures. At 300K, the O- Pd2Se3 monolayers show a large thermal electronic conductivity and a power factor of about 18 (16) times that of O-Pd2S3 along the x (y) directions. At temperatures higher than 600K, these properties become more dominant in O-Pd2S3. The acquired power factor of O-Pd2S3 (O-Pd2Se3) is much higher than that of the 1T phase of PdS2 (1T PdSe2 and penta-PdSe2). By just knowing the group velocities of the three acoustical modes, we were able to determine the minimum lattice thermal conductivity, which is 0.207 W/(m.K) for both systems. If properly exploited, these characteristics make the O-Pd2Q3 monolayers excellent candidates for the fabrication of novel ultrathin electronic and TE nanodevices at ambient and higher temperatures.